MODIFICATION OF CAST HIGH-SPEED STEEL WITH BROMINE

V. T. Svishchenko

1. Introduction

Cast cutting tools are finding ever wider application in industry. In many plants, cast tools have been used for a number of years.

Numerous studies (carried out by T. A. Lebedev, A. N. Dobrovidov, A. M. Rosenberg, I. A. Revis, S. A. Kazeev, L. P. Ermoshkin, and other Soviet scholars) have clarified a number of theoretical and practical questions concerning the use of cast tools: economic advantages, cutting properties, manufacturing technology, optimum chemical compositions, optimum heat treatment, etc.

The advantages of using cast tools consist, first, in saving costly high-speed steel; second, in shortening and reducing the cost of the tool-manufacturing cycle; and, third, in the possibility of using scrap and worn-out tools made of high-speed steel for remelting.

It should also be added that, in many operations, cast tools have greater durability than tools made of forged steel.

However, cast tools also have very substantial shortcomings in comparison with forged tools.

These shortcomings include, above all, the increased brittleness of cast tools, which in many cases makes their practical use difficult. A second substantial shortcoming is the nonuniformity of the cutting properties of cast tools when used on different steels. The durability of cast tools decreases sharply with an increase in the toughness of the metal being machined, the greatest decrease in durability being observed under impact loading (milling). It may be considered established that in milling operations cast tools perform better than forged tools on relatively brittle steels of the ShKh—15 type, and much worse than forged tools on relatively tough steels of the types steel 45, steel 30KhGSA. The indicated shortcomings hinder the widespread introduction of cast tools into production; therefore, the elimination of these shortcomings is a very important practical problem. As has already been noted, many researchers are working on the problem of cast tools. Extensive research work on cast tools has been conducted for a number of years at the Department of Metallography of the Tomsk Polytechnic Institute under the supervision of Professor-Doctor A. N. Dobrovidov.

It should be noted that everywhere, with the exception of the Tomsk Polytechnic Institute, tools are cast in ceramic dry molds.

At the Tomsk Polytechnic Institute, a method has been developed and implemented on a production scale for casting cutting tools in metallic ...

metal rotating molds, i.e., under pressure. The advantages of this method consist, first, in the high productivity of the process and, second, in a further shortening of the tool-manufacturing cycle, since a high-speed-steel tool cast in a chill mold is hardened during the cooling of the casting, and heat treatment of the tool consists only of a triple tempering.

When hardening takes place during the cooling of the casting, i.e., from the maximum possible temperature, a larger amount of alloying elements than usual passes into the solid solution (austenite), as a result of which the cutting ability of the cast tool is increased. This is a third, very substantial advantage of the method of casting the tool in a chill mold.

The present investigation is part of the general work on cast tools being carried out at the department. The investigation was undertaken with the aim of improving the cutting properties of cast high-speed steel by affecting the structure of primary crystallization of the steel by modification. The work mainly investigated the effect of small additions of boron on the properties of cast high-speed steel with different tungsten contents. The following questions were studied:

1. The effect of boron on refinement of the structure of cast steel.
2. The effect of boron on the cutting properties of cast steel.
3. The effect of boron on changes in the microhardness of the individual structural constituents of cast steel (the dark constituent, the light constituent, and the network of the carbide eutectic).
4. The effect of boron on changes in the amount and composition of the structural constituents of cast steel.

At the same time, the effect of changes in tungsten content on the structure and cutting properties of cast high-speed steel was investigated; a small study was made of the effect of changing the geometry of the cutting part on the durability of cast knives for inserted face mills.

It was assumed that modification with boron would increase the toughness and cutting properties of cast high-speed steel and would make possible the successful use of cast tools in milling operations on tough steels.

2. Casting of the tool

Most of the experimental work was carried out at one of the plants, with the Department of Metallography of TPI organizing production of cast tools at that plant. As the experimental tool, knives for inserted face mills and three-sided milling cutters of two sizes—35 × 24 × 8.5 and 23 × 14 × 6—were cast. The steel was melted in a coreless induction furnace. Boron was added (in an amount from 0.0017% to 0.053%) in the form of ferroboron or in the form of a complex alloy with boron in the crushed state at the last moment of melting before the metal was poured into the molds.

The steel was preliminarily deoxidized with aluminum, calculated for an addition of 0.09% aluminum. When heats with a boron addition in the form of ferroboron were made, worn tools of steel grades RF-1 and EI-262 were remelted.

Heats with a boron addition in the form of a complex alloy were cast only from steel of grade RF-1. Casting was carried out on a centrifugal machine with a vertical axis of rotation and an adjustable number of revolutions, into metal molds. The temperature of the metal at the moment of pouring was maintained within 1450–1500°. The temperature of the liquid metal was controlled with an optical pyrometer. The surface of the metal was protected by slag (ordinary glass). The duration of melting was 8–12 minutes. Carbon burn-off was compensated by adding “high-speed cast iron”—carburized high-speed steel. Ferroboron was prepared by us in the metallographic laboratory of TPI by the aluminothermic

method and contained 3% boron and 4–5% aluminum. The chemical composition of the complex alloy with boron was:

Tables 1–3 give the chemical composition of heats with the addition of boron in the form of ferroboron; Table 4 shows the calculated boron content in heats with the addition of boron in the form of a complex alloy.

Table 1

Chemical composition of heats with low tungsten content (7.62–12.60%)

Table 2

Chemical composition of heats with medium tungsten content (13.01–15.85%)

Table 3

Chemical composition of heats with high tungsten content (16.45–18.25%)

Table 4

Calculated boron content in heats with addition of boron in the form of a complex alloy

Since, when casting a high-speed-steel tool in a chill mold, hardening of the tool takes place during the cooling of the casting, the heat treatment of the cast cutters consisted only of triple tempering. For cast cutters modified with boron (and unmodified ones), the following tempering schedule was selected:

1st tempering at a temperature of 600°,
2nd ” ” 620°,
3rd ” ” 630°.

The holding time at the tempering temperature was one hour. The indicated tempering schedule ensures that the normal hardness of the cast cutters is obtained within the range of 63–66 Rockwell units \((R_c)\).

Cutters of all the heats listed in Tables 1–4, after heat treatment, were tested by cutting. In addition to the listed heats, a large number of unmodified heats of high-speed-steel cutters with different tungsten contents and approximately equal contents of the other alloying elements were cast. These heats were cast specifically for the study of the microstructure of cast high-speed steel with different tungsten contents and were not tested by cutting.

3. Cutting tests

Cutting tests of cutters of all heats were carried out in the plant’s cutting laboratory. Heats of cutters of size 23×14×6 with boron added in the form of ferroboron were also tested under production conditions in the plant’s tool shop.

The laboratory tests were carried out in face milling of mild steel with a tensile strength of 61 kg/mm², using a single-tooth milling cutter, without cooling. For each cutter size, the standard cutting conditions adopted at the plant for the corresponding size of forged-steel cutters were used.

Cutting conditions in testing cutters of size 35×24×8.5: cutting speed—26.2 m/min, milling depth 3 mm, feed per tooth 0.26 mm.

Cutting conditions in testing cutters of size 23×14×6: cutting speed—26.7 m/min, milling depth 3 mm, feed per tooth 0.18 mm.

Simultaneously with the testing of cast cutters, cutters (size 35×24×8.5) made of forged steel of grade RF-1 were tested by cutting.

The blunting criterion adopted was flank wear equal to 1.5 mm.

Our main task was to compare the cutting properties of cast steel modified with boron with the cutting properties of cast unmodified steel when operating under identical conditions. Comparison of endurance

the cast knives with the life of knives made of forged steel was a secondary task, since this was required by the conditions for introducing the cast tool at the plant. These same conditions for introduction determined the choice of material for testing and the geometry of the cutting part of the knives made of forged steel, which were tested with the geometry accepted at the plant, i.e., under the existing plant operating conditions for knives made of forged steel.

At the time the cutting tests were carried out, work at TsSfAN on determining the optimum geometry for cast tools and tools made of forged steel had not yet been completed. From this work it was known to us only that the geometry of the cutting part of a cast tool should differ from the geometry of the cutting part of a tool made of forged steel, and that the life of a cast tool increases with an increase in the clearance angles. In order to obtain the maximum life of cast knives satisfying the condition for introduction of the cast tool at the plant, we carried out a small investigation of the effect on life of changing (increasing) the clearance angles. It was established that increasing the clearance angles to the values indicated in Table 5 (the geometry obtained experimentally) increases the life of cast knives by 85–95%.

Thus, a special geometry was selected for the cast knives, after which knives of all heats were tested with one and the same (selected) geometry of the cutting part.

Table 5

Geometry of the cutting part of inserted face-milling cutters with cast knives and knives made of forged steel

Cutters of size \(35 \times 24 \times 8.5\) were tested both with the geometry used at the plant and with that obtained experimentally. Cutters of size \(23 \times 14 \times 6\) were tested only with the special geometry.

The results of laboratory tests of the knives are given in Tables 6–8. The life values given in Tables 6–8 were obtained as averages from several tests of knives of one and the same heat. Table 6 shows the life of cast knives of size \(35 \times 24 \times 8.5\) from modified heats with a low tungsten content (7.62–12.60%), as well as heat No. 1 without boron, heat No. 7, and knives made of forged steel. We did not have a cutting-tested heat of knives of size \(35 \times 24 \times 8.5\) without boron and with a low tungsten content; therefore, the life of knives from modified heats with a low tungsten content is compared with the life of the unmodified heat No. 1 containing 18.09% tungsten. Apparently, in this case a small error is admitted, since the knives of the unmodified heat No. 10, whose chemical composition is very close to the optimum composition of cast unmodified high-speed steels for face-milling-cutter knives (1. With a content of 0.62–0.85% carbon and 10–13% tungsten, and 2. With a content of 1% carbon and 15% tungsten—according to

according to TsFAN data), have a durability approximately equal to that of knives of heat No. 1. The reduction in the strength margin of knives of heat No. 10, which have a smaller size (23×14×6), than knives of heat No. 1 (35×24×8.5), is compensated by reducing the cutting conditions for knives of the smaller size.

Table 6

Durability of knives of size 35×24×8.5

Table 7

Durability of cast knives of size 23×14×6 from heats with a tungsten content of 13.0–15.85%, tested with special geometry

The data presented in Tables 6–8 show that an addition of boron in an amount from 0.01% to 0.025% leads to a sharp increase in the durability of cast knives made of high-speed steel with various tungsten contents. Durability decreases when larger (than indicated) amounts of boron are added. The durability of cast knives from heats modified with the optimum amount of boron (in the form of ferroboron) increases by 3–4 times in comparison with the durability of knives from unmodified heats.

Table 8

Service life of cast cutters measuring 23 × 14 × 6 from heats with 16.45–18.25% tungsten, tested with special geometry

Cast cutters from unmodified heats have increased brittleness in comparison with cutters from heats modified with boron in the form of ferroboron, and therefore showed a comparatively low service life. The service life of cutters from unmodified heats Nos. 1 and 10, indicated in Tables 6–7, was obtained as an average from the largest number of tests. Moreover, a considerable number of tests of cutters from these heats ended on the first passes because of chipping of the cutting edge, which was never observed when testing cutters from modified heats. The relative service life of cast cutters from unmodified heats, in comparison with the service life of cutters made of forged steel, is approximately 50% when testing cast cutters with special geometry and 27.5% when testing cast cutters with the geometry used for cutters made of forged steel.

The best heats, with an optimum (or nearly optimum) addition of boron, when working on tough steel with a tensile strength of 61 kg/mm², have a service life equal to approximately 170% of the service life of cutters made of forged steel when testing cast cutters with special geometry, and about 90% of the service life of cutters made of forged steel when testing cast cutters with the geometry adopted for forged cutters. It should also be noted that modification with boron in the form of ferroboron sharply increases the service life of cast tools made of high-speed steel of various chemical compositions. This circumstance is of exceptional importance in the production of cast tools from high-speed-steel scrap, which in most cases consists of different grades (as a rule, steels of grades RF-1 and EI-262, which are difficult to distinguish by spark). When remelting high-speed-steel scrap, it is practically very difficult to obtain cast steel of a definite (optimum) chemical composition.

As already noted, production cutting tests were carried out at the plant on cast cutters 23 × 14 × 6, modified with boron in the form of ferroboron. The tests were conducted in the tool shop in three-sided milling of tough steels under the same conditions as with cutters made of forged steel. The production tests mainly confirmed the results of the laboratory tests.

Of all the cutters tested in the plant’s cutting laboratory from heats with boron added in the form of a complex alloy, only heat No. 11, with an addition of 0.0017% boron, had a high service life (but one considerably lower than with the addition of boron in the form of ferroboron). The cutters of all the other heats with boron added in the form of a complex alloy proved brittle and showed low service life. Thus, it was established that the use of a complex alloy with boron for modifying cast high-speed steel is less advisable than the use of ferroboron.

4. Metallographic Analysis of Melts

The microstructure of cast knives made of high-speed steel with various tungsten contents, cast in a chill mold (modified and unmodified), consists of three structural constituents—a light constituent, a dark constituent, and a network of carbide eutectic. Study of the structure of the unmodified melts made it possible to draw the following conclusions.

1. The structure of the unmodified melts is characterized by a relatively coarse grain.

2. The volume occupied by the network of carbide eutectic increases with increasing tungsten content in the steel. The carbide-eutectic network is the coarser, the higher the tungsten content in the steel (at equal carbon content).

3. The amount of the dark constituent increases with increasing tungsten content. At a tungsten content above 14% (and about 0.9% carbon), the dark constituent is distributed in the form of a relatively coarse (black) network within each grain, which in the center of the grain passes into continuous regions, the number and size of which increase with a further increase in tungsten content. The fact that the dark (black) network within the grain is the dark constituent is indicated by the sharp decrease in microhardness at the places where the black network is concentrated, and by the circumstance that the network passes directly into continuous regions of the dark constituent in the center of the grain.

Study of the structure of the modified melts made it possible to note the following regularities.

1. Modification with boron in the form of ferroboron causes a sharp decrease in the grain size of cast high-speed steel with various tungsten contents (approximately from 8% to 19% tungsten) and promotes the formation of a very fine network of carbide eutectic. The grain size decreases up to a certain (optimal) boron concentration and increases again with a further increase in the boron concentration. The greatest refinement of the structure is caused by the addition of boron in the form of ferroboron in an amount of 0.015–0.025% boron.

2. In the structure of melts with a high tungsten content (up to 19%) modified with boron, continuous regions of the dark constituent are absent. Under the influence of the boron addition, the dark constituent is distributed within the grain in the form of a very fine network (black after etching with a 10% solution of nitric acid in ethyl alcohol).

3. The volume occupied by the network of carbide eutectic increases under the influence of modification with boron, but to a considerably lesser degree than under the influence of an increase in the tungsten content in unmodified steel.

4. Addition of boron in the form of a complex alloy (in an amount of 0.0017% boron) likewise leads to grain refinement and eliminates the possibility of formation of continuous regions of the dark constituent; however, in this case a significantly smaller effect is achieved than in the case of adding boron in the form of ferroboron.

5. Addition of large amounts of the complex alloy with boron (more than 0.0017% boron, or more than 0.125% alloy) leads to the formation of a large number of foreign (nonmetallic) inclusions located along the grain boundaries, which causes increased brittleness of the cast tool.

6. Judging from the results of cutting tests, the best cutting properties are possessed by melts with fine grain, a fine network of carbide eutectic, and distribution of the dark constituent (within the grain) in the form of a very fine (black) network.

5. Measurement of Microhardness

The microhardness of the structural constituents of the cast steel was measured on a PMT-3 instrument. The impressions were measured at a magnification of \(\times 487\). The load on the diamond pyramid was in all cases 20 grams. In each specimen up to twenty measurements were made of an individual structural constituent, and then the average microhardness values were taken. Before microhardness measurement the specimens were subjected to electropolishing. In all heats it was possible to measure only the microhardness of the light constituent. Since only the unmodified heat No. 1 had continuous areas of the dark constituent, the microhardness of the dark constituent could be measured in this heat alone. The average microhardness value of the dark constituent is equal to \(450\ \text{kg}/\text{mm}^2\). Since the dark constituent has comparatively low hardness (and strength), the presence of large continuous areas of the dark constituent in the structure of cast steel may adversely affect the cutting properties of the tool.

Nor was it possible in all heats to measure the microhardness of the carbide-eutectic network, especially in the modified heats, where there were few large accumulations of eutectic. The microhardness of the eutectic was measured at places where it accumulated, where both carbide inclusions and austenite (martensite) inclusions were clearly visible under the microscope in the form of very fine alternating plates. A large number of both carbide and austenitic (martensitic) inclusions entered into the impression of the diamond pyramid. We were able to establish that the microhardness of the eutectic in the modified heats, especially in the as-cast state (before tempering), is higher than that of the unmodified heats. After tempering, the microhardness of the eutectic increases considerably in the unmodified heats and less considerably in the modified heats. At the same time, after tempering the microhardness of the eutectic in the modified heats (\(1000—1100\ \text{kg}/\text{mm}^2\)) remains higher than the microhardness of the eutectic in the unmodified heats (\(800—1000\ \text{kg}/\text{mm}^2\)). This indicates that the composition of the eutectic in the modified and unmodified heats is different. Apparently, in the eutectic of the unmodified heats there is more austenite (retained in the as-cast state) and fewer carbides.

Tables 9–11 give the average microhardness values of the light constituent of the heats.

Table 9

Microhardness of the light constituent of heats with low tungsten content

Table 10

Microhardness of the light constituent of heats with medium tungsten content

Table 11

Microhardness of the light constituent of heats with a high tungsten content

As can be seen from the data presented, the microhardness of the light constituent in heats modified with boron decreases up to a certain (optimum) boron content and increases again with a further increase in the boron content. The highest values of the microhardness of the light constituent are found in the unmodified heats. The microhardness of the light constituent in all heats decreases at the sites where the network of the dark constituent accumulates. The greatest decrease in the microhardness of the light constituent at the sites where the network of the dark constituent accumulates is observed in the unmodified heats, where the network of the dark constituent is coarser (thicker). Thus, in unmodified heats of cutters there is greater nonuniformity in mechanical properties within the grain. The high hardness of the light constituent of unmodified heats causes (together with the coarse grain and the coarse network of carbide eutectic) increased brittleness of the cutters of these heats when working under impact loading (milling).

In heats modified with boron (with an optimum or nearly optimum amount of boron), there is no abrupt change in hardness within the grain (owing to the distribution of the dark constituent in the form of a very fine network and the absence of continuous regions of the dark constituent). The microhardness of the light constituent of heats modified with boron is considerably lower than that of unmodified heats and approaches the microhardness of martensite in forged steel, which, according to Lebedev [1], is approximately 680–700 units. The microhardness of the network of carbide eutectic in the modified heats is higher than that in the unmodified heats, but in the modified heats the network of carbide eutectic is very fine. All this leads to the fact that cutters made from heats modified with boron, in comparison with cutters made from unmodified heats, possess increased toughness; therefore they do not chip out when working under impact loading (during milling) and have high durability.

In heats with the addition of boron in the form of a complex alloy, the microhardness of the structural constituents also changes (in comparison with unmodified heats), but this change is expressed much less sharply than in heats with the addition of boron in the form of ferroboron.

From a comparison of the microhardness of the light constituent of unmodified heats No. 10 and No. 1 (see Tables 10–11) with different tungsten contents (13.01% and 18.09%), it may be concluded that, under the influence of an increase in the tungsten content in the steel, a decrease in the microhardness of the light constituent also occurs, but it is much less significant than under the influence of modification with boron. The decrease in the microhardness of the light constituent is explained by a decrease in the concentration of carbon in the gamma solid solution, part of which passes into the carbide phase, increasing the amount of the latter; since the microhardness of the light constituent decreases under the influence of modification with boron to a considerably greater degree than under the influence of an increase in the tungsten content in cast unmodified steel, it follows from this that, under the influence of modification, a larger amount of carbon passes from the gamma solid solution into the carbide-

phase than under the influence of an increase in the tungsten content in the steel; consequently, the amount of the carbide phase should increase to a greater extent, as is also indicated by the increase in the hardness of the carbide eutectic under the influence of modification.

At the same time, as already noted, the volume occupied by the carbide eutectic under the influence of modification with boron increases to a considerably lesser extent than under the influence of increasing the tungsten content in unmodified steel. This can be explained only by the fact that the carbide phase of cast steel modified with boron consists mainly of chromium carbides (\(\mathrm{Cr_7C_3}\)), the volume of which, according to Lebedev [2], is three times smaller than the volume of the complex carbide \(\mathrm{Fe_3W_3C}\). The carbide phase of cast unmodified steel, however, consists mainly of the complex carbide (\(\mathrm{Fe_3W_3C}\)); therefore, with a smaller amount of carbon entering the carbide phase, the volume of the carbide phase of unmodified steel increases to a greater extent.

The change in the microhardness of the structural constituents of cast steel (and the change in the volume occupied by the carbide phase) makes it possible to conclude that, under the influence of modification with boron, a redistribution of alloying elements occurs between the phases of cast high-speed steel. The concentration of carbon and chromium in the light constituent decreases, while the concentration of tungsten increases (owing to the smaller formation of complex carbide). Correspondingly, in the carbide phase the concentration of chromium and carbon increases, while the concentration of tungsten decreases.

6. Conclusions

1. Modification with boron up to a certain (optimal) concentration causes a sharp decrease in the grain size of cast high-speed steel and promotes the formation of a very fine network of carbide eutectic. The dark constituent is distributed within the grain in the form of a very fine (black) network.

2. The volume occupied by the carbide eutectic in cast high-speed steel modified with boron increases in comparison with that in unmodified high-speed steel of one and the same chemical composition. Increasing the tungsten content in cast unmodified steel also leads to an increase in the volume of the carbide eutectic, but a considerably sharper one than in the first case.

3. Under the influence of modification with boron, the microhardness of the structural constituents of cast high-speed steel changes in both the cast and tempered states. The microhardness of the light constituent decreases as the addition of boron is increased (in steel of one and the same chemical composition) up to a certain (optimal) concentration, and increases again with a further increase in the boron concentration. The microhardness of the light constituent also decreases with increasing tungsten content in the steel, but much less significantly than under the influence of modification with boron. The microhardness of the carbide eutectic of cast high-speed steel modified with boron is higher than that of unmodified high-speed steel.

4. The microhardness of the light constituent of unmodified cast high-speed steel (about \(900\ \mathrm{kg/mm^2}\)) is much higher than the microhardness of martensite in forged steel (about 700 units). At the same time, in places where the coarse network of the dark constituent is concentrated, as well as in places of continuous areas of the dark constituent, the microhardness drops sharply, since the microhardness of the dark constituent (\(450\ \mathrm{kg/mm^2}\)) is somewhat lower than the microhardness of martensite in forged steel. The high hardness of the light constituent, as well as the presence of a coarse (thick) network of carbide eutectic,

causes increased brittleness of the cast unmodified tool. The presence of a coarse network of the dark constituent (inside the grain) and continuous regions of the soft (and not strong) dark constituent likewise reduces the durability of the cast unmodified tool.

1. The microhardness of the light constituent of boron-modified high-speed steel (750–800 kg/mm²) approaches the microhardness of martensite in forged steel. The dark constituent is distributed (within each grain) in the form of a very fine network, at the places where it accumulates the microhardness decreases only slightly. The greater uniformity in hardness of the main metallic mass, as well as the presence of a very fine network of carbide eutectic, ensures increased toughness of the cast boron-modified steel.

2. The data on the change in microhardness of the structural constituents and the change in the volume occupied by the carbide phase make it possible to conclude that modification with boron causes a redistribution of alloying elements between the phases of cast high-speed steel. The concentration of carbon and chromium decreases in the gamma solid solution and increases in the carbide phase. The concentration of tungsten in the gamma solid solution increases, and in the carbide phase decreases.

3. In milling tests on tough steel with an ultimate strength of 61 kg/mm², the cast end-mill cutters of unmodified heats had increased brittleness. There were cases of chipping of the cutting edge during operation.

4. Modification with boron increases the durability of cast cutters by 3–4 times in comparison with the average durability of cutters of unmodified heats when operating under identical conditions. During the cutting tests there was not a single case of chipping of the cutting edge of cutters from modified (with ferroboron) heats.

5. Modification with boron makes it possible to obtain sufficiently high durability of a cast tool made of high-speed steel with a considerable difference in chemical composition (within the range from the chemical composition of grade EI-262 to the chemical composition of steel grade RF-1); therefore, modification with boron should be especially recommended in the production of cast tools from high-speed-steel scrap, when confusion in the composition of the scrap is possible (RF-1 or EI-262).

6. The optimal addition of boron in the form of ferroboron to cast high-speed steel with various tungsten contents lies within the range of 0.015–0.025% boron, provided that the steel is deoxidized with aluminum, calculated for an addition of 0.09% aluminum.

7. The effect of an addition of boron in the form of a complex alloy is insignificant in comparison with the effect of an addition of boron in the form of ferroboron. The optimal addition of boron in the form of a complex alloy is equal to 0.0017% boron.

8. The durability of cast end-mill cutters increases significantly when the rear cutting angles are increased.

9. Modification with boron in the form of ferroboron and the use of a rational geometry of the cutting part make it possible successfully to use cast tools in milling operations on tough steels.

Literature

1. Lebedev T. A., Revis I. A. Structure and Properties of Cast Tools Made of High-Speed Steel. Mashgiz, 1949.